It is necessary to provide projection exposure systems with a very high numerical aperture in order to achieve the highest resolutions in microlithography. Light is coupled into the resist layer at very large angles. When this light is coupled in, the following occur: light losses because of reflection at the outer resist boundary layer and deterioration of the resolution because of lateral migration caused by reflections at the two boundary layers of the resist to the wafer and to the air (formation of standing waves).
The degree of fresnel reflection is then dependent upon the angle between the polarization direction and the reflection plane. The reflection vanishes when light having an electrical field oscillating parallel to the incident angle incidents at the brewster angle. This provides for optimal in-coupling into the resist while at the same time providing maximum suppression of the standing waves.
However, disturbances occur for light which is linearly polarized in one direction as described in European patent publications 0,602,923 and 0,608,572. Accordingly, the apparatus disclosed in these publications generate circularly polarized light which is coupled into the resist as the equivalent of unpolarized light. In this way, homogeneity is achieved over the entire image. However, a loss of efficiency is accepted because in eacl case, the locally perpendicular polarized light component is intensely reflected.
In European patent publication 0,602,923, it is alternatively suggested that linearly polarized light should be orientated in one direction relative to the orientation of a pattern to be imaged as already disclosed in German patent publication 1,572,195. The penetration via a multiple reflection takes place in the longitudinal direction of the structures and not in the direction of the critical resolution. The efficiency of the in-coupling or the reflection at the resist surface is however not homogeneous.
The effect of the polarization on the reflection at the resist layers and the significance of the fresnel coefficients is described in U.S. Pat. No. 4,899,055 directed to a method for measuring thickness of thin films.
U.S. Pat. No. 5,365,371 discloses a projection exposure apparatus for microlithography wherein a radially directed linear polarization of the light is introduced in order to prevent disturbances because of standing waves in the resist when generating images therein. Two different polarization elements are given, namely, a radial polarization filter composed of a positive cone and a negative cone. This filter is utilized in transmission and effects radial polarization for the reflection because of the fresnel equations. However, it is not disclosed how a complete polarization of the transmitted light is achieved. In the description of U.S. Pat. No. 5,365,371 and in claim 3 thereof, it is required in addition that both parts have different refractive indices. The transmitted part must then however be deflected and cannot: pass in a straight line.
U.S. Pat. No. 5,436,761 has a disclosure identical to that of U.S. Pat. No. 5,365,371 referred to above and includes a single claim wherein no condition is given for the indices of refraction. Furthermore, in claim 4 of U.S. Pat. No. 5,365,371, a plate having segments of radially orientated polarization filter foils is given as is known from U.S. Pat. No. 4,286,843 (see FIG. 19 and column 9, lines 60 to 68).
Both polarizers are polarization filters, that is, they lead to high light loss and are suitable only for an incoming light beam which is unpolarized or circularly polarized because, otherwise, an intense nonhomogeneity of the intensity would occur over the cross section of the exiting light beam.
In the example of FIG. 1 of U.S. Pat. No. 5,365,371, the deflecting mirror 17 causes a partial polarization and therefore the light beam exiting from the polarizer 21 is nonhomogeneous.
U.S. Pat. 5,365,371 discloses that the radial polarizer lies in the pupillary plane of the projection objective. A position of the radial polarizer in the objective is problematical because there, the tightest tolerances for an optimal image quality must be maintained.